refactor: use mnemonic in deployment script

docs: document BIP39 mnmemonic in README

README.md

@@ -142,6 +142,9 @@ Deploy to Anvil:
 $ forge script script/DeployFoo.s.sol --broadcast --fork-url http://localhost:8545
 ```
 
+For this script to work, you need to have a `MNEMONIC` environment variable set to a valid
+[BIP39 mnemonic](https://iancoleman.io/bip39/).
+
 For instructions on how to deploy to a testnet or mainnet, check out the
 [Solidity Scripting tutorial](https://book.getfoundry.sh/tutorials/solidity-scripting.html).
 

script/DeployFoo.s.sol

@@ -6,10 +6,16 @@ import { Foo } from "../src/Foo.sol";
 
 /// @dev See the Solidity Scripting tutorial: https://book.getfoundry.sh/tutorials/solidity-scripting
 contract DeployFoo is Script {
+    address internal deployer;
     Foo internal foo;
 
+    function setUp() public virtual {
+        string memory mnemonic = vm.envString("MNEMONIC");
+        (deployer,) = deriveRememberKey(mnemonic, 0);
+    }
+
     function run() public {
-        vm.startBroadcast();
+        vm.startBroadcast(deployer);
         foo = new Foo();
         vm.stopBroadcast();
     }